This application is based on Japanese Patent Application No. 35665/1998 filed on Feb. 18, 1998, 41744/1998 filed on Feb. 24, 1998, 46657/1998 filed on Feb. 27, 1998, 46658/1998 filed on Feb. 27, 1998, 55187/1998 filed on Mar. 6, 1998, and 55188/1998 filed on Mar. 6, 1998, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a three-dimensional (3D) measuring apparatus for measuring the profile of an object in a non-contact way by projecting a light beam on the object.
2. Description of the prior art
A non-contact type 3D measuring apparatus (3D camera) called a range finder is capable of high-speed measurement as compared with the contact-type version and therefore finds such applications as data input into a CG system or a CAD system, the human body measurement and visual recognition of a robot.
A well-known measuring method suitable for the range finder is a light projection method. According to this method, an object is optically scanned to produce a distance image (3D image) based on the principle of trigonometry. This method is a kind of an active measuring method for raster scanning of an object by projecting a beam-like reference light. The raster scanning is divided into a method in which the main scanning is carried out unidirectionally from left to right, for example, and a method in which the scanning from left to right is alternated with the scanning in reverse direction (reciprocal main scanning). The distance image is a mass of pixels indicating three-dimensional positions of a plurality of points on the object. The calculation for determining a distance image from the pickup information is carried by a range finder or an external information processing system such as a computer system.
Generally, the measurement information obtained by the range finder is input to the information processing system on-line or off-line using a storage medium and subjected to a predetermined processing such as analysis, change, storage, and display.
The range finder is possibly used for displaying a distance image with a display unit connected thereto or as a 3D video camera for displaying on the monitor the position and profile change of a moving object by repeating the measurements. In such a case, the distance image or the one-frame measurement data constituting the base of forming the distance image is temporarily stored in a memory, and required to be read out in each frame period of display. In the process, the memory is controlled by simply writing the data in the order of generation sequentially at a predetermined sampling period concurrently with the raster scanning. An address pointer is incremented at sampling periods.
Such a method of memory control, however, requires rearrangement (rewrite) or the read address control to meet the line-by-line display requirement. Especially for improving the measurement rate in reciprocal main scanning, the address designation for read operation is complicated.
The reference light reflected on the object enters the photo-detection surface of a photo-electric conversion element. The spot position on the photo-detection surface corresponds to the angle of incidence, and therefore the distance from the object can be calculated by detecting the spot position.
In the prior art, in order to receive the light reflected from each position in the scanning range, a 2D image pickup device such as a CCD area sensor or a PSD (position sensing-type photo-detector) or a pseudo-2D photo-detection surface is formed by use of an arrangement of a plurality of 1D pickup devices.
As described above, in the 3D measurement in which the light beam is projected, the larger the photo-detection surface for photo-electric conversion, the lower the S/N ratio of the detection signal. This is because the percentage that the spot represents of the photo-detection surface is reduced. For some devices, the response speed is also decreased with the increase in the photo-detection surface area.
On the other hand, a look-up table method is useful for distance calculation. Specifically, the distance data for various angles of incidence are determined using a calculation formula based on the specifications including the focal distance of the photo-detection lens or the relative positions of the projection side and the light-detection side, and the data thus obtained are stored in memory. The address of the memory is designated by the output of the photo-detection device indicating the angle of incidence and thus the desired distance data are read out. The distance data can alternatively be calculated by arithmetic operation instead of using the look-up table (LUT).
The provision of the zooming function to the range finder increases the freedom of framing for determining the scanning range. Also, it is convenient if measurement is possible from a point either far or near from the object.
The zooming function, however, changes the focal length of the photo-detection lens which is one of the measurement conditions. Specifically, the value of the coefficient in the calculation formula of the distance data is changed. When obtaining the distance data by LUT, therefore, the LUT is required for each of various focal lengths. This poses the problem of a large-capacity memory required for storing as many LUTs as the zooming stages. The larger the required memory capacity is, the higher the resolution of measurement.
Also, in the measurement by raster scan with a light beam described above, the time required for measurement is determined by the rate of main scanning. The subsidiary scanning can be slower than the main scanning. A higher speed of measurement is desirable. Especially, for a moving object, measurement is required to be repeated at shorter intervals of time. However, the speed of the mechanism for deflecting the light beam (such as a galvanometer mirror) has its own upper limit.
The provision of the zooming function in the range finder for changing the angle of visibility for light detection, on the other hand, improves the freedom of framing for determining the scanning range (the visual field). Also, it is convenient if the measurement of a predetermined resolution is possible from a point either far or near from the object of measurement.
Also, a method using a slit light as a reference light ray (sometimes called the light cut-off method) is widely known. In this method, a slit-like reference light of a length corresponding to the main scanning range is projected for linear scanning. As compared with the spot scanning by projecting a beam-like reference light, therefore, the scanning time per frame is shorter.
As described above, the PSD is sometimes used in place of an ordinary CCD image sensor as a photo-electric conversion device. Specifically, in this method, a pseudo-2D photo-detection surface is formed by arranging a multiplicity (for example, 128) of 1D PSDs having a band-shaped photo-detection surface along the direction of main scanning. Each PSD outputs an analog signal corresponding to the position of the light entering the band-shaped photo-detection surface thereof. The use of the PSD which eliminates the use of the charge accumulation permits higher-speed scanning than the CCD. At the same time, the resolution in the direction of subsidiary scanning is remarkably improved (improved infinitely theoretically).
With the above-mentioned measurement with the PSD array, however, the resolution in the direction of main scanning is defined by the number of lines in which the PDSs are arranged. The higher the number of PDSs arranged, the higher the cost. Also, the head amplifier is required for each PSD, and therefore the number of PSDs that can be arranged on a predetermined size of the circuit is limited.